Readme: Format table for packagist.org

Author: tangrufus

README.md

@@ -95,10 +95,9 @@ $ php-matrix --mode=minor-only  ">=7.2 <8.4"
 ### `--source`
 
 Available sources:
-- `auto`: Use `offline` in `minor-only` mode. Otherwise, fetch from [php.net](https://www.php.net/releases/index.php)
+- `auto` *(default)*: Use `offline` in `minor-only` mode. Otherwise, fetch from [php.net](https://www.php.net/releases/index.php)
 - `php.net`: Fetch releases information from [php.net](https://www.php.net/releases/index.php)
 - `offline`: Use [hardcoded releases information](resources/all-versions.json)
-[default: "auto"]
 
 <dl>
   <dt><code>auto</code> (default)</dt>